The present invention relates to the field of chemistry and more particularly to a novel process of preparing (2β,3α,5α,16β,17β)-17-acetoxy-3-hydroxy-2-(4-morpholinyl)-16-(1-pyrrolidinyl)androstane, which is typically used as an intermediate in the preparation of rocuronium bromide.
Neuromuscular blocking agents (NMBA) are muscle-relaxing drugs having similar muscle paralyzing activity as the naturally occurring alkaloid d-tubocurarine, used for hunting for centuries by natives of South America who extract it from curare, a vine native to the Amazon Basin. These agents, used in modern clinical practice since 1942, are known to interrupt transmission of nerve impulses at the skeletal neuromuscular junction and cause skeletal muscle contraction to cease.
NMBAs are routinely used as anesthesia adjuvants in the operating theatre, to enable endotracheal intubation and to facilitate mechanical ventilation, i.e., relaxation of vocal cords, jaw muscles etc., to facilitate surgery, i.e., providing generalized muscle relaxation so as to allow surgical access to body cavities, in particular the abdomen and thorax, without hindrance from voluntary or reflex muscle movement, as relaxants to prevent the violent muscle movements associated with electroconvulsive therapy treatment, and for surgery under convulsive conditions. Typically, administration is performed intravenously by injection of a suitable dosage form.
Based on their mechanisms of action, NMBAs are divided into two categories: depolarizing and non-depolarizing.
Depolarizing neuromuscular blocking agents bind to nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction in a way similar to that of the endogenous neurotransmitter acetylcholine, but in a noncompetitive mode. They stimulate an initial opening of the ion channel, producing contractions known as fasciculations. However, since these drugs are cleared relatively slowly by cholinesterase enzymes, as compared to the very rapid hydrolysis of acetylcholine by acetylcholine esterases, they bind for a much longer period than acetylcholine, causing persistent depolarization of the end-plate and hence cause a neuromuscular block. Succinylcholine (Suxamethonium) is a classical example of a depolarizing NMBA.
Non-depolarizing neuromuscular blocking agents compete with acetylcholine for binding to muscle nAChRs, but unlike depolarizing NMBAs, they do not activate the channel. Rather, non-depolarizing NMBAs block the activation of the channel by acetylcholine and hence prevent cell membrane depolarization, and, as a result, the muscle becomes flaccid.
Most of the clinically-used NMBAs belong to the non-depolarizing category. These include tubocurarine, atracurium, (cis)atracurium, mivacurium, pancuronium, vecuronium, rapacuronium and rocuronium.
Hence, 1-[(2β,3α,5α,16β,17β)-17-acetoxy-3-hydroxy-2-(4-morpholinyl)-androstan-16-yl-]-1-(2-propenyl)pyrrolidinium bromide, also known by the name rocuronium bromide, is a neuromuscular blocking agent having a steroidal skeleton as shown hereinunder.

Rocuronium bromide, which is presently marketed in the North America under the brand name ZEMURON® (rocuronium bromide) and elsewhere under the brand name ESMERON® (rocuronium bromide), is used in clinical practice since 1994 as a non-depolarizing neuromuscular blocking agent. It is known for its remarkable rapid yet controllable onset, depending on dose and intermediate duration. Rocuronium bromide is indicated for patients as an adjunct to general anesthesia, to facilitate both rapid sequence and routine tracheal intubation, and to provide skeletal muscle relaxation during surgery or mechanical ventilation.
Processes for preparing rocuronium bromide are described in U.S. Pat. Nos. 4,894,369 and 5,817,803, and in Tuba, Z. et al. (2002), “Synthesis and structure-activity relationships of neuromuscular blocking agents.” Curr. Med. Chem. 9(16): 1507-36, which are all incorporated by reference as if fully set forth herein.
U.S. Pat. No. 5,817,803 describes a process of preparing rocuronium bromide, which involves acetylation of (2α,3α,5α,16β,17β)-2,3-epoxy-17-hydroxy-16-(1-pyrrolidinyl)androstane (Compound A, Scheme 1 below), so as to obtain Compound B (Scheme 1), and reacting Compound B with morpholine to yield a compound having the Formula I below, which is also referred to herein throughout as Compound I, as illustrated in Scheme 1 below. As is further detailed hereinbelow, Compound I is a known intermediate in the synthesis of rocuronium bromide.

U.S. Pat. No. 5,817,803 fails to provide any experimental data and therefore the efficiency of the process described therein, in terms of, for example, chemical yield and purity, is not demonstrated. A skilled artisan, however, can assume that reacting Compound B with morpholine, under the conditions described in this patent, may result in a side product formed between the morpholino and the acetoxy group at position 17 and therefore affect the efficiency of the process.
U.S. Pat. No. 4,894,369 teaches a process of preparing rocuronium bromide, in which the two final steps include a selective acetylation of (2β,3α,5α,16β,17β)-2-(4-morpholinyl)-16-(1-pyrrolidinyl)androstan-3,17-diol (a compound having Formula II below, which is also referred to herein throughout as Compound II) at position 17 to give (2β,3α,5α,16β,17β)-17-acetoxy-3-hydroxy-2-(4-morpholinyl)-16-(1-pyrrolidinyl)androstane (Compound I), followed by the allylation of the nitrogen on the pyrrolidine ring with allyl bromide, to thereby provide rocuronium bromide.

Such a selective acetylation at position 17, in which the hydroxyl at position 3 is left intact, is not a simple reaction to perform and hence adversely affects the process efficiency in terms of yield and product purification.
Indeed, the teachings of U.S. Pat. No. 4,894,369 demonstrate that the conversion of Compound II to. Compound I is inefficient. The crude product, Compound I, is obtained by treating Compound II with 1.13 equivalents of acetyl chloride in dichloromethane at room temperature for 18 hours, and purifying the crude product by column chromatography to give the desired Compound I at a yield of 48%. Therefore. this conversion technique reduce the overall efficiency of the process substantially and is further limited by the need to purify the product by column chromatography, which is known as a very inconvenient, expensive and time consuming procedure, especially in commercial scale.
U.S. Pat. No. 4,894,369 further teaches that performing the procedure described above in the presence of a 6.3 molar excess of acetyl chloride, results in the corresponding 3,17-diacetate(2β,3α,5α,16β,17β)-3,17-diacetoxy-2-(4-morpholinyl)-16-(1-pyrrolidinyl)androstane (a compound having Formula III below, which is also referred to herein throughout as Compound III). According to the teachings of this patent, Compound III is thus obtained as an off-white froth at 67% yield, while no additional purification of this compound was reported.

Reacting Compound III with methanol showed an undesired selectivity, according to U.S. Pat. No. 4,894,369, yielding as the main product (2β,3α,5α,16β,17β)-3-acetoxy-17-hydroxy-2-(4-morpholinyl)-16-(1-pyrrolidinyl) androstane (a compound having Formula IV above, which is also referred to herein throughout as Compound IV). Thus, the acetoxy group at position 3 is retained while the acetoxy group at position 17 is transformed to a hydroxyl group.
The greater susceptibility of the acetoxy group at position 17 to hydrolysis, as compared with the acetoxy group at position 3, is further supported in “Proceedings of the 4th Symposium on the Analysis of Steroids, Peos, Hungary, 1990, pp. 261-268”. This article describes studies performed on a controlled hydrolysis of the two acetoxy groups (at the 3 and 17 positions) of vecuronium bromide (NORCURON®(vecuronium bromide)), a structurally related compound which is also used as a non-depolarizing neuromuscular blocking agent, as is illustrated below.

According to the teachings of this reference, treating vecuronium bromide with methanol and sodium hydroxide yielded a mixture of the 3,17-dihydroxy corresponding compound and the 3-acetoxy-17-hydroxy corresponding compound. Under less drastic conditions, reacting vecuronium bromide with methanol gave the 3-acetoxy-17-hydroxy compound as a single product. These results, which indicate that under convenient hydrolytic conditions, the 3-acetoxy group is stable while the 17-acetoxy group is easily hydrolyzed, are in accord with the teachings of U.S. Pat. No. 4,894,369 cited above.
The above-cited article further teaches that under intense UV irradiation of a methanolic solution of vecuronium bromide, the selectivity is reversed such that the 17-acetoxy group is retained and the 3-acetoxy group is hydrolyzed. However, a process utilizing intense UV irradiation is highly impractical on a commercial scale.
There is thus a widely recognized need for, and it would be highly advantageous to have, an improved process for to the preparation of Compound I, devoid of the above limitations.